1. Field of the Invention
The present invention relates generally to the fields of biomedical engineering and surgery. More specifically, the present invention provides a device and methods for improving the ease with which anatomic structures can be anastamosed.
2. Description of the Related Art
There has been an effort recently to identify biocompatible molecules which can be used as a “tissue solder”. Biomolecules such as fibrin, elastin, albumin have been or are used to “glue” tissue-to-tissue. A number of patents describe the “activation” of these biomolecules to form “welds” through irradiation, often in the form of laser radiant energy, but sometimes in the form of ultrasound or radiofrequency waves. The applied energy is believed to denature the molecules, which then adhere to one-another, or cross-link thereby effecting a union between the tissues.
Over the past fifteen years, a significant amount of scientific research has focused on using laser heated “solder” for “welding” tissues such as blood vessels (1-2). Research has been done on laser tissue welding with albumin solders which are an improvement over conventional suture closure because it offers an immediate watertight tissue closure, decreased operative time, especially in microsurgical or laparoscopic applications, reduced trauma, and elimination of foreign body reaction to sutures, collagen-based plugs and clips. The procedure has been enhanced with the use of advanced solders, strengthening structures, concurrent cooling, and added growth factors as disclosed, for example, in U.S. Pat. No. 6,221,068.
Use of lasers for tissue welding appeared very promising, however, the techniques have certain limitations. The laser energy must be manually directed by the surgeon which leads to operator variability. Additionally, the radiant energy is not dispersed evenly throughout the tissue. The high energy at the focal point may result in local burns and the heating effect drops off rapidly at a small distance from the focal point. Finally, lasers are expensive and currently cannot be miniaturized easily.
U.S. Pat. No. 5,669,934 describes a method for joining or restructuring tissue by providing a preformed film or sheet of a collagen and/or gelatin material which fuses to tissue upon the application of continuous inert gas beam radiofrequency energy. Similarly, U.S. Pat. No. 5,569,239 describes laying down a layer of energy reactive adhesive material along the incision and closing the incision by applying energy, either optical or radiofrequency energy, to the adhesive and surrounding tissue. Further U.S. Pat. No. 5,209,776 and U.S. Pat. No. 5,292,362 describe a tissue adhesive that is intended for use principally in conjunction with laser radiant energy to weld severed tissues and/or prosthetic material together.
U.S. Pat. No. 6,110,212 describes the use of elastin and elastin-based materials which are biocompatible and can be used to effect anastomoses and tissue structure sealing upon the application of laser radiant energy. Both U.S. Patent Application No. 20020045732 and U.S. Pat. No. 6,221,335 teach joining living tissues by using laser radiant energy to heat a “solder” consisting of protein, water and a compound which absorbs the laser radiant energy, preferably indocyanine green. This solder is optionally in the form of a tubular structure which can more easily be applied when vascular anastomoses are required. The stated benefits, inter alia, are the biocompatible and ubiquitous nature of elastin.
U.S. Pat. No. 6,302,898 describes a device to deliver a sealant and energy to effect tissue closure. It also discloses pre-treating the tissue with energy in order to make the subsequently applied sealant adhere better. PCT Publication WO 99/65536 describes tissue repair by pre-treating the substantially solid biomolecular solder prior to use. U.S. Pat. No. 5,713,891 discloses the addition of bioactive compounds to the tissue solder in order to enhance the weld strength or to reduce post-procedure hemorrhage.
U.S. Pat. No. 6,221,068 discloses the importance of minimizing thermal damage to the tissue to be welded. The method employs pulsed laser irradiation followed by cooling the tissue to nearly the initial temperature between each heating cycle. U.S. Pat. No. 6,323,037 describes the addition of an “energy converter” to the solder mixture such that optical energy will be efficiently and preferentially absorbed by the solder which subsequently effects a tissue weld.
Common problems exist throughout the prior art. These include tissue damage due to uneven heating, unknown and/or uncontrollable thermal history, i.e., time-temperature profile, and relatively high cost. It is notable that a consistent means of treatment and control are desirable. The Code of Federal Regulations, 21 CFR 860.7(e)(1), establishes that there is “reasonable assurance that a device is effective when it can be determined, based upon valid scientific evidence, that in a significant portion of the target population, the use of the device will provide clinically significant results.” Devices that cannot be shown to provide consistent results between patients, or even within a patient upon multiple use, will have minimal utility and may not be approved, if approved, for broad use. Beyond devices, it is generally desirable to develop medical products with critical controls that can deliver precise results.
Inductive heating (3) is a non-contact process whereby electrical currents are induced in electrically conductive materials (susceptors) by a time-varying magnetic field. Generally, induction heating is an industrial process often used to weld, harden or braze metal-containing parts in manufacturing where control over the heating process and minimized contact with the workpiece are critical.
Basically, radiofrequency power is coupled to a conducting element, such as a coil of wire, which serves to set up a magnetic field of a particular magnitude and spatial extent. The induced currents or Eddy currents flow in the conductive materials in a layer referred to as the skin depth δ (m), given by:δ=√/(2ρ/μΩ),where Ω is frequency (rads/s), ρ is resistivity (ohm-m) and μ is the permeability (Webers/amp/m) which is the product of μo the permeability of free space and μr the relative permeability of the material.
The magnetic permeability of a material is quantification of the degree to which it can concentrate magnetic field lines. Note, however, that the permeability is not constant in ferromagnetic substances like iron, but depends on the magnetic flux and temperature. The skin depth at room temperature at 1 MHz electromagnetic radiation in copper is 0.066 mm and in 99.9% iron is 0.016 mm.
The consequence of current flowing is Joule, or I2R, heating. The skin-depth formula leads to the conclusion that, with increased frequency, the skin depth becomes smaller. Thus, higher frequencies favor efficient and uniform heating of smaller components. In certain situations localized heat can also be generated through hysteresis losses or frictional heating, referred to as dielectric hysteresis heating in non-conductors, as the susceptor moves against physical resistance in the surrounding material. Consideration of Joule heating alone results in a formula for the power-density P (W/cm3) in the inductively-heated material:P=4πH2μoμr f M,where H is the root-mean-square magnetic field intensity (A/m), f is frequency (Hz), M is a power density transmission factor (unitless) which depends on the physical shape of the heated material and skin depth and diameter of the part to be heated (4-5).
M, which is equal to the product of F and d/d where F is a transmission factor and d is the diameter of the part, can be shown to be maximally about 0.2 when the object diameter is 3.5 times the skin depth and when certain other assumptions are made. Thus, for a given frequency there is a diameter for which the power density is a maximum; or equivalent, there is a maximum frequency for heating a part of a certain diameter below which heating efficiency drops dramatically and above which little or no improvement of heating efficiency occurs. It can also be shown that the power density of inductively heated spheres is much higher than solid spheres of the same material.
There are only a few examples of the use of inductive heating in the medical literature. The oldest example of use of therapeutic inductive heating is in hyperthermia of cancer, whereby large metallic “seeds” are inductively heated using a coil external to the body (6). Smaller seeds were used where small biocompatible dextran magnetite particles in magnetic fluid was used to treat mouse mammary carcinoma by hyperthermia (7). U.S. Patent Application Ser. No. 2002/0183829 describes inductively heating stents made of alloys with a high magnetic permeability and low curie temperature for the purpose of destroying smooth muscle cells in restenosing blood vessels. A more recent report described the diagnostic use of induction heating to heat nanocrystals coupled to DNA in order to locally denature DNA for the purpose of hydridization (8.) The literature is deficient in descriptions whereby biomolecules are heated through induction. U.S. Pat. No. 6,348,679 discloses compositions used in bonding two or more conventional materials where the interposed composition consists of a carrier and a susceptor, which may be at least in part composed of certain proteins. However the applications apply to conventional substrates such as films or wood.
The inventors have recognized an increased need in the art for a precision device and improved methods of joining tubular, planar or irregular-surfaced tissues to other tissue structures or to dressings. Further, the prior art is deficient in devices and methods for minimally-invasive methods that use electromagnetic energy to controllably alter a biocompatible structure thereby making it adhere to tissue through molecular alterations and/or mechanical shrinkage. The present invention fulfills this longstanding need and desire in the art.